Odd and even
- Even numbers end in 0, 2, 4, 6 or 8. They split into two equal groups.
- Odd numbers end in 1, 3, 5, 7 or 9. There is always one left over.
| Sum or product | Answer is always |
|---|---|
| even + even | even |
| odd + odd | even |
| odd + even | odd |
| odd × odd | odd |
| anything × even | even |
Prime numbers
A prime number has exactly two factors: 1 and itself.
- Primes up to 30: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29
- 2 is the only even prime number.
- 1 is not prime: it only has one factor.
Square and cube numbers
| Name | How to make it | First few |
|---|---|---|
| Square numbers | a number × itself | 1, 4, 9, 16, 25, 36, 49, 64, 81, 100, 121, 144 |
| Cube numbers | a number × itself × itself | 1, 8, 27, 64, 125 |
We write 5 × 5 as 5² (“5 squared”) and 2 × 2 × 2 as 2³ (“2 cubed”).
Watch out! 9 is odd but it is not prime: 9 = 3 × 3. Always check whether a small number divides into it.
Top tip: to test if a number is prime, try dividing by 2, 3, 5 and 7. If none of them go in exactly (and the number is under 100), it’s prime!
👆 Prime hunt
Tap all the prime numbers
🎮 Number detectiveQuestion 1 of 5